The Fostex Model M22RP
ms stereo printed ribbon microphone
M/S Stereo Microphone
In this design, the process of generating the stereo signal is accomplished by the addition and subtraction of MID and SIDE signals. The M22RP has two capsules. The Mid capsule is a cardioid element in the front of the mic which picks up the on-axis signal (mono). This signal is L + R. The Side capsule is a bidirectional element, positioned 90° in relation to the cardioid element, which picks up the Left and Right signals at the same time, but out of phase with each other. This signal is L – R. By combining these two signals, the user can decode from 180° stereo all the way to mono—without any phase irregularities.
The Left channel is made by adding the MID (L + R) to the SIDE (L – R), thus canceling R: (L + R) + (L – R) = (L + L) + (R – R) = 2L = Left. The Right channel is made in the same manner, except that the MID (L + R) is now added to the out of phase SIDE (-L + R), thus canceling L: (L + R) + (-L + R) = (L -L) + (R + R) = 2R = Right. The stereo signal produced by these combinations is equivalent to matched cardioid microphones placed 180° apart. If no SIDE signal is added, the result is pure mono. As incremental amounts of SIDE signal are added, the stereo separation increases to a maximum of 180° when the MID and SIDE signals are of equal strength.
One major benefit of this design is that the user can adjust the stereo spread before committing the sound to a recording; or the MID and SIDE signals can be recorded on separate tracks, and be reconstructed into the stereo perspective later on.
Another advantage is that when the operator sums the Left and Right, there are no comb filtering or phase cancellation effects. Summing simply removes the SIDE signals from the equations:
L = (L + R) + (L – R)
R = (L + R) + (–L + R)
L + R = Mono
(L + R) + (L – R) + (L + R) + (–L + R)
2 (L + R) = Mono
This aspect is important for broadcast applications. AM Stereo, FM Stereo and TV Stereo are received in mono at many locations. With the Fostex M22RP, Mono summing may occur anywhere in the audio chain with complete phase integrity.
Thus the M22RP is really three microphones in one. In addition to the interaction between the two elements, each may be used separately. The MID element alone is the equivalent of the Fostex M11RP, an excellent cardioid microphone. The SIDE element alone is the same as that in the Fostex M88RP, a highly versatile bidirectional, or Figure 8 mic.
Microphone and matrix box courtesy of Jeff Rudisill
M22RP Applications
The Fostex M22RP is ideal for broadcasters, not only in the studio for stereo announcing and interviewing, but also for ENG and on-location work. An external power supply is not required, so the M22RP fits anywhere a single channel mic goes. The M22RP is, of course, exceedingly well-suited for the recording studio, and thanks to its rugged construction, does equally well on the road. It has been used successfully on instruments such as drums, pianos and vibes, which normally require more than one mic, and therefore create phasing problems.
RP stands for Regulated Phase, a patented technology which has been under development by Fostex engineers for the past ten years. This unique transducer design has produced a family of microphones that exhibit extremely low distortion, excellent transient response, and wide dynamic range. In the RP system, a flat, thin diaphragm is driven with absolute phase uniformity, in true piston motion. Utilizing integrated circuit manufacturing techniques, a fine aluminum coil is etched directly onto a surface of an extremely thin polyester film diaphragm. This assembly is then suspended in a powerful magnetic field formed of magnet pairs which have opposing polarities. Rare earth samarium cobalt magnets produce a magnetic flux density ten times that of Alnico, which fact contributes to the fast transient response of the RP element. Simplicity of design and impeccable construction techniques produce a family of microphones for the complete range of professional applications—in the studio, on the air, or up on stage.
Soon after their introduction, the Fostex mics were being called “Printed Ribbon” Microphones, so fondly did their warmth resemble the beautiful but frail ribbon mics of yesteryear. (The reference to “printed” is to one patented aspect of Fostex RP Technology, wherein the voice coil is etched directly onto the diaphragm.) But the reference to “ribbon” carried a negative connotation regarding durability. However, Fostex RP Mics have a history which verifies a ruggedness quotient equal to, or better than, the best dynamics. In the RP system, a flat, thin diaphragm is driven with absolute phase uniformity, in true piston motion. Eighty per cent lighter than the typical dynamic element, the RP element has the fast transient response of a condenser.
Fostex RP Microphones use a magnetic design in place of the polarizing voltage design of a condenser. The advantage of this fact can be seen in the ability of the RP design to be applied to any other pattern, with full integrity. The basic RP pattern is bi-directional, or “Figure 8.” The acoustic air mass load is the same, in fact, on both sides of the diaphragm. Given this reciprocal pick-up pattern as a basic building block, the RP design can be predictably controlled all the way from a stereo mic to a super cardioid. The high magnet flux on the surface of the RP diaphragm produces outstanding control, reduces diaphragm break-up, and results in lower distortion overall. Fostex mics have less than 0.2 per cent total harmonic distortion at 130 dB SPL—a figure unmatched by some of the finest condensers.
Lower Distortion
The patented Fostex RP design permits capsule construction which has identical front/back response. This design reduces second harmonic distortion to almost unmeasurable levels, lower than some of the best condenser microphones. At 100 dB, RP microphones have less than 0.01 per cent distortion, equal to many high-quality electronics. At 130 dB SPL, RP mics have less than 0.2 per cent distortion. The high magnetic flux, the flat diaphragm, and the spiral voice coil all contribute to complete diaphragm control at all sound pressure levels. This patented design results in operating levels so high that they are normally associated with laboratory mics.
Quoted from the Fostex RP Technology Microphones brochure,
produced during April, 1986. Appreciation is expressed to Mr.
Matt Suzuki, Fostex Product Support, for his assistance.
Dear Prof. Coutant,
Thank you for your message and also for the great web site that you created.
As Matt has explained to you, the Model M22RP has long been discontinued and unfortunately we can no longer obtain any spare parts from the factory. This means once it gets badly damaged, there is no one to repair the microphone. We also find this to be a great shame.
The Fostex RP-series microphones were produced solely relying on the state-of-art technique and skill of the dedicated engineers. The mics were totally hand-made and fine-tuned one by one. However, apparently those engineers were all retired and the current factory structure cannot maintain such a traditional method of production as it would require highly subjective expertise and know-how.
We believe the M22RP would have (and does have, as you know) a high standard compared to any of today’s microphones with modern technology. As a matter of fact, we still occasionally receive sales inquiries for the models even now.
We are very much obliged for the M22RP continuously being one of your favorite microphones.
Best regards
Masaki Shimmachi
Fostex Technical Support
Download the sales brochure for this mic.
1.7 megabytes, pdf
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